1. Field of the Invention
The present invention relates, generally, to a clutch assembly, and more specifically, to a start-up clutch assembly for translating torque between a prime mover and a transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the “transmission.” Engine torque and speed are converted in the transmission in accordance with the tractive power demand of the vehicle. Hydrokinetic devices, such as torque converters, are often employed between the internal combustion engine and its associated automatic transmission for transferring kinetic energy therebetween.
Torque converters typically include impeller assemblies that are operatively connected for rotation with the torque input from an internal combustion engine, a turbine assembly that is fluidly connected in driven relationship with the impeller assembly, and a stator or reactor assembly. These assemblies together form a substantially toroidal flow passage for kinetic fluid that circulates in the torque converter. Each assembly includes a plurality of blades or veins that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly and the turbine assembly. When the stator assembly is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, when the stator assembly freewheels in the direction of rotation of the impeller and turbine assemblies, there is no torque multiplication and the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. In the absence of a fully engaged lock-up clutch, torque converter slip exists when the speed ratio is less than 1.0 (RPM input>RPM output of the torque converter). This inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in parasitic losses that decrease the efficiency of the entire power train. More specifically, the operating efficiency of the converter during start-up is relatively low. It varies from a zero value at stall to a maximum value of approximately 80-85% at the coupling point. The coupling point occurs at the transition from the torque multiplication mode to the coupling mode when the torque multiplication ratio is unity.
In addition to the problems with efficiency, torque converters of the type known in the related art occupy substantial space in the driveline assembly between the transmission gearing and the engine. Torque converters typically define relatively large diameters when compared to the transmission gearing. Further, the torque converter has a substantial rotating mass that must be accelerated by the engine during start-up of the vehicle during forward drive or in reverse drive. The effective mass of the converter necessarily includes the mass of the hydraulic fluid that circulates in the torus circuit defined by the converter impeller, the turbine, and the stator assembly.
On the other hand, frictional clutches have been also employed in the related art to selectively connect a source of rotational power, such as the crank shaft of an internal combustion engine and its flywheel, to a driven mechanism, such as a transmission. The frictional clutches of the type that have been employed in the related art overcome the disadvantages associated with reduced efficiencies, parasitic losses, relatively large effective mass and the space that is occupied by torque converters used for the same purpose. In an automotive context, clutches used for this purpose are often referred to as “start-up” clutches. Clutches of this type typically include a clutch pack that is operatively supported between a drive and driven member of the clutch assembly. The clutch pack typically incorporates a first set of clutch disks operatively connected to a drive member and a second set of clutch disks that are alternately disposed between the first set of disks and are operatively connected to a driven member. In operation, the two sets of disks are operatively forced together to form a frictional connection to transfer torque between the drive member and the driven member. The drive member is operatively connected to the torque input from the prime mover. The driven member is operatively connected to the input shaft of the transmission.
In addition, some start-up clutches include a series connected, torsional-vibration damper disposed between the clutch pack and the output to the input of the transmission. The torsional-vibration damper serves as an elastic coupling between the two main components of drive train of a vehicle (i.e., the engine and the transmission). Such devices reduce or otherwise prevent vibrations from being transmitted from the engine to other parts of the drive train.
While start up clutch assemblies having a clutch pack to transfer torque of the type generally known in the related art have performed reasonably well for their intended purposes, some disadvantages remain. More specifically, the disks of the start up clutches generate a good deal of heat as they are brought into engagement. Furthermore, as the overall structure of start up clutch assemblies move to smaller more efficient designs, the clutch packs are generally smaller requiring them to dissipate even greater quantities of heat energy. Some attempts have been made to improve the supply and flow of cooling oil within the clutch housing. However, changes in the supply of cooling oil within the clutch housing to compensate for heat have often caused other issues relating to the pressure balance of cooling oil between the drive and driven side components. Uncompensated pressure differences of this nature can cause uncontrolled slipping or uncontrolled engagement of the clutch disks.
In particular, a hydraulically controlled piston is actuated to cause the engagement of the clutch pack. Generally speaking, the actuating piston is dynamically balanced as it has oil on both sides. The actuating side is supplied with oil under actuating pressure and the opposite side has a supply of oil for cooling the friction surface of the friction elements. However, during the start-up procedure, conditions arise in which the actuating piston becomes partially unbalanced due to centrifugal force acting upon the cooling oil supply by the driven components. This is due to the fact that, during the start-up procedure, the two halves of the system (drive and driven) operate at different rotational speeds. The components of the input or drive side are rotating at engine speed, with the output or transmission side generally stationary. As a result, the fluid on the drive side of the actuating piston will be given an increase in pressure by the centrifugal effect of the drive member components.
If, from a rotational standpoint, the actuating piston is positioned on the driven or transmission side, the increase of forces from the oil area on the motor side reduce the effective engagement force causing uncontrolled and undesired slipping in the clutch pack. If, the actuating piston is positioned on the drive or engine side, it moves with engine speed and the centrifugal increase of oil pressure may cause the clutch pack to engage too rapidly.
Accordingly, there remains a need in the related art for a start-up clutch assembly that provides a supply and flow of cooling oil to the clutch disks to efficiently dissipate the frictional heat output while providing compensation for the increase in cooling oil pressure due to the centrifugal force of the drive components.